fix(hts): IG conformance — multilingual display wording, designation order, hierarchical $expand

[smunini]

Why

The nightly HTS Terminology IG Conformance job (run 27456608477) went red with no new commits to main. That workflow checks out HL7/fhir-tx-ecosystem-ig unpinned and downloads validator_cli.jar latest on every run, so the upstream commit 741e6e7 "add additional tests for language and secondary displays" (2026-06-12 05:43 UTC, between the green 06-12 and red 06-13 runs) is what turned it red.

It added 7 tests — 6 exercise PR #149's new multilingual code, 1 is a brand-new feature. Failures were identical across all 4 matrices (sqlite/postgres × R4/R5).

What

Multilingual display validation (validate_code.rs) — when a display is valid in the CodeSystem's default language but the requested displayLanguage has no designation, quote the supplied display and use "…which is a valid display for the default language".
Fixes validation-right-de-en, validation-right-de-ende-N, validation/simple-{code,coding,codeableconcept}-good-language-none.

Designation ordering (expand.rs apply_display_language) — front-insert the synthesised preferredForLanguage designation (the rotated primary display) instead of pushing it last. The HL7 validator sorts designations by language (stable), so a same-language explicit designation was beating it to designation[0].
Fixes language-xform-en-multi-de-hard.

Hierarchical-by-default \$expand (new feature) — when a request omits both excludeNested and hierarchical and the inline ValueSet wholly includes is-a CodeSystem(s) with real parent/child edges, return a nested expansion (tx.fhir.org behaviour). Adds the code_system_is_hierarchical trait method (sqlite + postgres). I audited the whole IG suite: this gate flips exactly vs-expand-versionless — the ~100 other excludeNested-omitting expand tests stay flat.
Fixes vs-expand-versionless.

Note: default-nesting is wired for inline valueSet composes (what the validator sends). URL-resolved ValueSets keep the flat default — correct for every current fixture, and documented inline as a scope boundary.

Testing

• Reproduced each failure locally against the actual upstream fixtures, then confirmed each fix.
• Full HTS sqlite suite green (607 lib + integration); postgres compiles; cargo fmt + clippy (sqlite & postgres, CI flags) clean.
• Adds regression tests expand_is_a_codesystem_nests_by_default and expand_is_a_codesystem_flat_with_exclude_nested.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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